Liquid Electro-Photography (LEP) printing devices form images on print media by placing a uniform electrostatic charge on a photoreceptor and then selectively discharging the photoreceptor in correspondence with the images. The selective discharging forms a latent electrostatic image on the photoreceptor. Ink comprising charged colorant particles suspended in imaging oil is then developed from a binary ink development unit on to the latent image formed on the photoreceptor. The image developed on the photoreceptor is offset to an image transfer element, where it is heated until the solvent evaporates and the resinous colorants melt. This image layer is then transferred to the surface of the print media being supported on a rotating impression drum.
Non-productive print cycles, referred to herein as null cycles, may be scheduled to occur before, during or after normal printing sessions. Such null cycles may be included, for example, to maintain synchronization between different subsystems of the printing device. For example, a null cycle may be included between print jobs, during a substrate change, while waiting for another subsystem to finish an operation, or while waiting for a temperature of a component of the printing device to stabilize.
During null cycles, latent images are not formed on the photoreceptor or transferred to the photoreceptor or image transfer element. The lack of ink transfer during null cycles can damage the photoreceptor and the image transfer element and reduce print quality. Therefore, in order to protect the photoreceptor and the image transfer element, some LEP systems perform so-called wet null cycles, in which a binary ink development unit transfers imaging oil, but not charged ink particles, to the photoreceptor. The transferred imaging oil helps to lubricate and protect the photoreceptor and the image transfer element.